The present invention relates to an improved process for the production of a liquid mixture of the isomers of di-(4-aminocyclohexyl)-methane by hydrogenation of di-(4-aminophenyl)-methane ("MDA"). The liquid isomer mixture obtainable in accordance with the invention contains from 15 to 25% by weight and preferably from 18 to 23.5% by weight of the trans-trans isomer.
A liquid isomer mixture of the type noted above is generally required for the production of an isocyanate derivative which in turn is liquid at room temperature (approximately 10 to 25.degree. C.).
In general, the hydrogenation of MDA leads to an isomer mixture in which the cis-cis isomer, the cis-trans isomer and the trans-trans isomer occur together and in which the trans-trans content approaches the equilibrium concentration of around 50% by weight. An isomer composition with the relatively low trans-trans isomer content mentioned above (i.e., from 15 to 25% by weight) can be produced by the complicated conversion process described in German Auslegungschrift 1,593,293. There are other known processes which allow for the preparation of an isomer mixture of di-(4-aminocyclohexyl)-methane containing a specific amount of the trans-trans isomer (see, e.g., U.S. Pat. Nos. 3,766,272, 3,644,522, 3,155,724, and 3,153,088). European Patent 324,190 describes a hydrogenation process for MDA which produces hydrogenation products wherein the trans-trans content corresponds to the desired value. The process requires the use of special catalysts and the maintenance of certain reaction conditions. However, this process has the disadvantage that the overall yield of di-(4-aminocyclohexyl)-methane is unsatisfactory. In addition, some of the hydrogenation or conversion processes mentioned must be carried out in the presence of ammonia or high-grade solvents.
Accordingly, the problem addressed by the present invention was to provide a new, simplified and industrially workable process in which the trans-trans isomer content would be in the desired range mentioned above with a high overall conversion. This problem has been solved by the process according to the invention.